Receptors for gut peptides

Most gut peptides exert their effects through G protein-coupled receptors, a family of about 700 membrane proteins, 87 of which are presently known to have peptide ligands. Three additional gut peptide receptors are not G protein-coupled receptors but regulate intracellular cyclic GMP accumulation. The aim of this review is to illustrate how the sequencing of the human genome and other recent advances in genomics has contributed to our understanding of the role of peptides and their receptors in gastrointestinal function. Recent discoveries include the identification of receptors for the peptides motilin and neuromedin U, and new physiological ligands for the PTH2 receptor, the CRF2 receptor and the growth hormone secretagogue receptor. Knockout mice lacking specific peptide receptors or their ligands provide informative animal models in which to determine the functions of the numerous peptide–receptor systems in the gut and to predict which of them may be the most fruitful for drug development. Some peptide–receptor signalling systems may be more important in disease states than they are in normal physiology. For example, substance P, galanin, bradykinin and opioids play important roles in visceral pain and inflammation. Other peptides may have developmental roles: for example, disruption of endothelin-3 signalling prevents the normal development of the enteric nervous system and contributes to the pathogenesis of Hirschsprung disease.